1. Field of the Invention
The present invention relates to a manufacturing method of a compressible printing layer and a manufacturing method of a blanket for printing.
2. Description of the Related Art
Four manufacturing methods of a compressible layer of a compressible rubber blanket, which is an example of the blanket for printing, are known to the art, including (1) a method using an impregnated paper sheet, (2) a salt leaching method, (3) a foaming agent method, and (4) a microcapsule method. In recent years, a blanket equipped with a compressible layer manufactured by the microcapsule method attracts attentions.
Jpn. Pat. Appln. KOKAI Publication No. 6-1091 discloses a method of manufacturing a compressible rubber blanket including at least one reinforcing layer. It is disclosed that a compressible layer comprising an oil resistant rubber mixture and microballoons contained in the oil resistant rubber mixture, said microballoon being formed of a methacrylonitrile-acrylonitrile copolymer having a thermal deformation temperature not lower than 120° C. and a pressure resistance of 40 kg/cm2, is made integral with a surface rubber layer, followed by vulcanizing simultaneously the compressible layer and the surface rubber layer under temperatures not lower than 120° C. so as to manufacture a compressible rubber blanket.
Concerning the vulcanizing method, the Japanese patent document quoted above teaches under the paragraph [0032] that a rubber blanket provided with an unvulcanized compressible layer is wound around a metal drum, and the metal drum is put inside a double-walled can having a steam of 150° C. introduced on the outside so as to heat the rubber blanket and finish the vulcanization.
However, if a rubber blanket provided with an unvulcanized compressible layer is wound around a metal drum, a high pressure in the thickness direction is applied to that portion of the rubber blanket which is positioned on the inner circumferential side. As a result, a buckling deformation is generated in the microcapsule contained in the rubber blanket on the inner circumferential side so as to increase the nonuniformity in the thickness of the compressible layer in the longitudinal direction. The nonuniformity in the thickness of the compressible layer brings about nonuniformity in the compressibility of the blanket so as to cause nonuniformity in the printability.
It should also be noted that, if the vulcanization is carried out by the method described under the paragraph [0032] of the Japanese patent document quoted above, the heat transmission to the rubber blanket is carried out substantially completely through the air, with the result that the heat diffusion rate to the rubber blanket is lowered so as to prolong the vulcanizing time.
On the other hand, Japanese Patent No. 2670188 discloses a manufacturing method of a compressible printing layer. According to the Japanese patent document quoted above, a plurality of thermoplastic microballoons having a melting temperature not lower than 135° C. are uniformly dispersed into the entire region of an elastomer matrix. Then, the surface of a base fabric layer is coated in a uniform thickness with the elastomer matrix containing the microballoons so as to form a coated base fabric layer. Further, the coated base fabric layer is subjected to a vulcanizing treatment at 80 to 150° C. for 1 to 6 hours so as to fix the positions of the thermoplastic microballoons having the high melting point within the matrix and, thus, to form a compressible layer. It is taught that the microballoons impart a uniform compression characteristics to the compressible layer.
It is taught in column 12, lines 42 to 50, of the Japanese patent document quoted above that, in the manufacturing method outlined above, the vulcanizing treatment is carried out by hanging the unvulcanized compressible layer within a furnace, by a continuous curing method such as a drum wrapping method or a rotocuring method, or by a curing method using a double belt press.
However, if the vulcanizing treatment is carried out by the hanging method, the heat transmission to the compressible layer is carried out through the air so as to lower the heating rate of the compressible layer, with the result that a long time is required for the vulcanizing treatment. In addition, force is exerted to the compressible layer at the fulcrum for supporting the compressible layer so as to make it difficult to render uniform the thickness of the compressible layer.
Also, if the vulcanizing treatment is carried out by the drum wrapping method, the thickness of the compressible layer is rendered nonuniform and a long vulcanizing time is required for the reason described above.
Further, in the rotocuring method, the vulcanizing treatment is carried out while applying pressure to the compressible layer, with an unvulcanized compressible layer held between a heated metal roll and a metal belt. On the other hand, in the double belt press method, the vulcanizing treatment is carried out while applying pressure to the compressible layer, with an unvulcanized compressible layer held between two heated belts. Each of these vulcanizing methods gives rise to the problem that it is difficult to make uniform the thickness of the compressible layer because it is difficult to maintain constant the pressure applied to the compressible layer.
Further, Jpn. Pat. Appln. KOKOKU Publication No. 6-59749 discloses a manufacturing method of a compressible blanket for printing. It is disclosed that a compressible intermediate layer prepared by dispersing microcapsules formed of, for example, an acrylonitrile-vinylidene chloride copolymer into an elastomer material is vulcanized for 1 to 12 hours under temperatures lower than the melting point of the microcapsule, e.g., at about 110 F (43° C.) to 170 F (70° C.), so as to fix the microcapsules within the elastomer material. In the method disclosed in the Japanese patent document quoted above, the vulcanization is carried out under low temperatures and, thus, used is an ultra vulcanization accelerator such as dithiocarbamate. Also, in the method disclosed in the Japanese patent document quoted above, cloth and a surface rubber layer are attached to the vulcanized compressible intermediate layer, followed by carrying out an essential vulcanizing treatment under temperatures of 132° C. to 160° C. so as to obtain a final product. In other words, the vulcanizing treatment is carried out in two stages in the method disclosed in the Japanese patent document quoted above.